1. Field of Invention
The present invention relates to a method for fabricating a semiconductor device. More particularly, the present invention relates to a method for fabricating a flash memory.
2. Description of Related Art
Electrically erasable and programmable read only memory (EEPROM) is currently one the most widely used memory devices in personal computers and electronic equipment. A memory cell in an early, conventional EEPROM comprises a transistor with a floating gate performing write, erase, and data storage operations during electrical shut down. This conventional memory cell typically occupies a large surface area. The data access speed is between 150 ns to 200 ns. A recently developed memory cell has a faster data access speed ranging from about 70 ns to 80 ns. This memory cell is called a flash memory by Intel Co.
FIG. 1 is a schematic, cross-sectional view of a conventional flash memory. A conventional flash memory cell comprises a transistor with a floating gate. Referring to FIG. 1, a semiconductor substrate 100 is provided. A field oxide layer 101 is formed on the substrate 100 by local oxidation (LOCOS) to define an active area. A tunneling oxide layer 102 is formed on the substrate 100. A floating gate 104 is formed on the tunneling oxide layer 102. A control gate 108 is formed over the floating gate 104. A dielectric layer 106 is formed between the control gate 108 and the floating gate 104. An N-type source/drain region is formed in the substrate 100 beside the floating gate 104. Oxide spacers 114 are formed on sidewalls of the floating gate 104 and the control gate 108 to protect the transistor with the floating gate from damage.
At present, a high-density flash memory fabrication becomes increasingly more important to satisfy requirements of high integration. However, in a conventional flash memory, the active area is defined by LOCOS isolation technology. The memory cell with a LOCOS isolation structure is hard to shrink the size so that device integration cannot be increased and also the capital expenditure is not reduced.